Many color printing technologies, in particular high speed printers, require that pixels of different colors be properly aligned with each other. Color plane registration (CPR) error can cause visible print artifacts if the error is greater than some threshold level, for instance, 50 microns. Although printers are manufactured to minimize CPR error, it tends to vary over time and when printing conditions change, such as when printing on different types of paper.
It is thus necessary to measure CPR error during the printing operation and adjust the printing process as necessary. Some CPR error measurement methods are manual. For example, one CPR error measurement method prints several lines with known offsets. An operator manually checks which lines are aligned with each other to determine the error.
To eliminate human intervention and associated innacuracies, other methods to measure CPR errors are automated. For instance, a combination of marks are printed, and an imaging device, such as a scanner or camera, captures an image of the printed marks. The image is then analyzed to determine the CPR error. The distance between printed marks or the optical density of the printed marks can be measured to determine CPR error, for example.
However, the imaging system for capturing the printed marks can introduce errors that in turn reduce the accuracy of the CPR error determination. For example, the imaging system that captures the image of the printed marks can introduce scaling and image skew errors, which are proportional to the distance between the features analyzed. Imperfections in the printed page can change the measured position of the printed mark. Variations in the image's background reflectance can adversely affect accuracy of optical density measurements. Further, to obtain the desired accuracy, such methods require a high resolution image, requiring a high resolution imaging scanner, which can add complexity and cost to the printing system.